1. Field of the Invention
The present invention relates to a tracking error detection device and a tracking error detection method for an optical disk apparatus.
2. Description of the Related Art
In reproducing an optical disk in which a series of pits or marks (hereinafter generally referred to as pits) is formed according to recording data in the form of coded modulation, the series of pits is irradiated with a laser beam. The data is reproduced on the basis of reflected light of the laser beam. Since it is necessary to track an objective lens so that the laser beam does not go out of the series of pits, a tracking error signal is obtained from the reflected light. For a conventional example of the tracking error detection device, there is the device based on a DPD TE (Differential Phase Detection Tracking Error) method (For example, paragraphs 0020 to 0028 in Japanese Patent Application KOKAI Publication No. 2001-34969).
In the DPD TE method, a quadrant photodetector detects the light which is reflected from an edge of the pit. Each of two detection signals obliquely opposite to each other relative to the center of the photodetector is added, a waveform of the added signals is equalized with an equalizer, and then the equalized signal is binarized with a binarizing circuit. Phase difference between two binarized signals (a pulse signal of a pulse width according to the series of pits) obtained in above-mentioned way is determined with a phase comparator to obtain a phase-difference detection signal. Since the phase-difference detection signal generates a pulse having a length according to the amount of phase error at timing when a code is inverted, a modulation component of the recording data contained in the phase-difference detection signal, i.e., a high frequency component relative to a servo band is removed by smoothing the phase-difference detection signal with a low-pass filter, and a tracking error signal according to deviation between the laser beam and the series of pits is generated by determining the difference between the two binarized signals with a differential amplifier having a gain of 1.
A modulation coding method of current DVD (Digital Versatile Disk) is an 8/16 modulation coding method. Run-length ranges from 2 to 10 (RLL (2, 10)). RLL (2, 10) means that at least two “0”s (ten “0”s at the maximum) are present between adjacent “1” and “1” and a minimum mark length of the pit corresponds to the three codes. The above-mentioned tracking error detection device correctly functions to the current DVD.
In recent years, standards of a next-generation DVD having a higher recording density have been proposed. In the next-generation DVD, a 4/6 or 8/12 modulation coding is adopted. The run-length ranges 1 to 10 (RLL (1, 10)). The 4/6 or 8/12 modulation code is one which can increase modulation efficiency to record the information in higher density. The 4/6 or 8/12 modulation code has a spectrum of a frequency component lower than that of the modulation code used in the DVD standard relative to a channel bit frequency. Therefore, noise in the high frequency component relative to the servo band remains in the tracking error signal, and the deviation between the laser beam and the pit can not be correctly detected.
Except the above-mentioned method, the tracking error detection method of the differential phase detection method also includes following methods:
1) A method in which the waveform of an output signal from each detection element is equalized with the equalizer without performing addition, the phase difference is independently calculated in back and forth direction of the detector corresponding to the series of pits, and the obtained phase difference signal is added to pass it through the low-pass filter;
2) A method in which the phase difference between a PLL (Phase Locked Loop) clock of an RF signal and an equalizing signal in which the waveform of each output signal from each detection element is equalized with the equalizer is calculated, and the obtained phase difference signal is added and subtracted to pass it through the low-pass filter. The same problem is generated in any method, because the modulation component of the phase difference signal, i.e., the high frequency component relative to the servo band is removed with the low-pass filter.
Thus, in the tacking error detection device using the conventional differential phase detection method, there is the problem that, when the minimum mark length is shortened in order to increase the recording density of the optical disk, the phase difference signal can not be smoothed and the modulation component of the recording code included in the phase difference signal, i.e., the high frequency component relative to the servo band remains as the noise.